Revert "Revert "Set Ruby version in Gemfile and .ruby-version by default""

This reverts commit a8f59044.

See discussion on https://github.com/rails/rails/pull/30016.

railties/CHANGELOG.md

@@ -2,6 +2,12 @@
 
     *Yuji Yaginuma*
 
+*   Add `ruby x.x.x` version to `Gemfile` and create `.ruby-version`
+    root file containing the current Ruby version when new Rails applications are
+    created.
+
+    *Alberto Almagro*
+
 *   Support `-` as a platform-agnostic way to run a script from stdin with
     `rails runner`
 

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -49,6 +49,10 @@ def readme
       copy_file "README.md", "README.md"
     end
 
+    def ruby_version
+      template "ruby-version", ".ruby-version"
+    end
+
     def gemfile
       template "Gemfile"
     end
@@ -253,6 +257,7 @@ def initialize(*args)
       def create_root_files
         build(:readme)
         build(:rakefile)
+        build(:ruby_version)
         build(:configru)
         build(:gitignore)   unless options[:skip_git]
         build(:gemfile)     unless options[:skip_gemfile]

railties/lib/rails/generators/rails/app/templates/ruby-version

+<%= RUBY_VERSION -%>

railties/test/generators/app_generator_test.rb

@@ -4,6 +4,7 @@
 
 DEFAULT_APP_FILES = %w(
   .gitignore
+  .ruby-version
   README.md
   Gemfile
   Rakefile
@@ -808,6 +809,17 @@ def test_gitignore_when_non_sqlite3_db
     end
   end
 
+  def test_inclusion_of_ruby_version
+    run_generator
+
+    assert_file "Gemfile" do |content|
+      assert_match(/ruby '#{RUBY_VERSION}'/, content)
+    end
+    assert_file ".ruby-version" do |content|
+      assert_match(/#{RUBY_VERSION}/, content)
+    end
+  end
+
   def test_version_control_initializes_git_repo
     run_generator [destination_root]
     assert_directory ".git"